September 24, 2010
By Phil W. Johnston
Testing ammunition is hard, hot--and fun--work
By Phil W. Johnston
The custom gelatin molds were constructed from .032-inch aluminum. Pam is used as a release agent.
Over the past year, I've looked at a variety of ammunition in these pages and I thought readers might be interested in the procedure and tools that I use to do this work. When I'm off to evaluate a load, I first try to lay my hands on a cross section of firearms that will extract the normal level of its performance. I strive to come up with ballistics that will mirror what you will see on the street or in the field while hunting. In this light, it would be foolish to run a particular handgun load through a rifle, or a handgun with an abnormal barrel length.
I like to assemble at least two firearms for most tests and often rely on three. Handguns equipped with 2 1/2-, 4- and 7 1/2-inch barrels will typically generate ballistics that you can expect to see in the real world. If I'm working with a round designed for a semi-auto, you'll see me make every attempt to run the load through several firearms. Sometimes, it is only possible to use one arm, however. If this is the case, that rig should exhibit middle-of-the-road ballistics. Most of the time a semi-auto featuring a 3 1/2- to five-inch barrel works well.
Typically, we fire at least five 5-shot groups from each arm to obtain the ballistic mix. I run everything at 25 yards 95 percent of the time, but if sightless belly guns are used, I reserve the right to move things up to 10 yards to keep everything within the confines of the bullet trap. Once in a while I might look at a round aimed more at long range. When that's the case, I move to my 100-yard range in the back yard.
Because a ballistic outing typically shoots up most of an afternoon, I constructed a homemade, radio-controlled target rolling system that will roll paper from one roller to another upon command from the shooting station. I used the guts of an old RollaBull Target system and a Cabela's target stand I bought on sale. It would be just as easy to construct this system from scratch. The system relies on a transmitter (one channel, at least), a receiver (must be on the same frequency as the transmitter) and a servo. Any Radio Control shop can supply these goodies.
The target roller and bullet trap. Birchwood Casey Shoot-N-C targets are stuck to the butcher paper unless the Ransom Rest is used. When the rest is used, group size only is important.
Our downrange servo swings a homemade aluminum arm that contacts a pressure switch that closes to allow current from a 12-volt battery to drive a small, salvaged IBM printer motor, which in turn powers the bottom or take-up roll. The top or supply roller is loaded with a large roll of freezer paper. I run the paper downward over the framework and onto the take-up roll. Sticking the required number of three-inch Birchwood Casey Shoot-N-C targets on the freezer paper and then re-rolling things onto the supply roller gets things set downrange. If I'm working with a rig in the Ransom Master machine rest, I don't mess with targets on the paper. Since the machine is directing things, I only need groups to measure. The Ransom Rest is mounted on a heavy, semifixed steel bench that can be moved into position when the system is going into service. I shoot from a farm shop into the 25-yard target paper and bullet trap through an open 14x16-foot door when the weather is palatable, and when it's not we shoot through an eight-inch hole cut in the door. When I'm eyeballing things, I slide a BR Pivot bench loaded with sandbags into position. If optics aren't being used, I slip on a good pair of competition shooting glasses to clear up the front sight.
Although I do live in the boondocks and have a clear look at miles and miles of wild blue yonder, I keep everything except the noise in my yard. Twenty years ago I hit the local foundry and laid out some hard-earned cash for a 3x3-foot sheet of half-inch T-1 armor plate. The local welder made a portable bullet trap by arranging the T-1 at a 45-degree angle with a sand trap at the bottom to catch spent slugs. The sides are constructed of quarter-inch mild steel, and the rig sits on top of some large steel wheels. This bullet trap will hold anything I've run into it, including some high-velocity rifles.
In use, the bullet trap is placed directly behind the paper roller for use at 10, 25 and 50 yards. I've got a separate bullet trap set up at 100 yards. The long-range trap consists of three huge old combine tires that are filled with gravel. This trap stands about 5x5 feet in diameter and will hold everything including the .50 BMG cartridge. As you've noted, I rely on a pair of Oehler 35P chronographs to catch the flying objects. I always arrange chronograph number one to catch the velocity of the bullets about 15 feet from the muzzle. The skyscreens for this vital machine are set up on four-foot spacing, and the skyscreens are equipped with Oehler's light kit for shooting in sun or shade. If I'm working with some interesting loads-- lightweight frangible .45 ACP loads, for instance--I often set up a second Oehler 35P chronograph at the target.
A gelatin block in position for a shot. Temperature is critical. The block must be as close to 39.2 degrees F as possible. The author calibrates the block using a Daisy air rifle that has proven to be consistent.
Because the groups open up downrange, I set up the second set of skyscreens two feet apart and don't put 'em in place until the first group has been fired. Even then, it's not uncommon to blast away a downrange skyscreen or two over the course of an afternoon. These poor downrange skyscreens are patched and held together with wire as we speak. Run a shot slightly low or wide and you're quickly fixing things downrange. I also run long cabinet lights over this set of skyscreens.
The Oehler 35P has the ability to memorize up to 20 shots for record, so the ballistic charts contain the average instrumental statistics for 20 of 25 shots. It's more difficult to get information downrange unless I'm working with a real tack driver (a revolver, most of the time), so I might only get 10 shots for record there. In addition to the average instrumental velocity, you'll see the ES or extreme spread--the spread in velocity from the highest to the lowest and the SD or standard deviation, which is the measure of the ammunition's uniformity. This is possibly the most important information in the chart; perhaps it should be first in the data. The SD is base
d on a mathematical formula that is "equal to the square root of the arithmetic average of the squares of the deviations from the mean in a frequency distribution" according to Webster's. While this may indeed be the most important figure in one of the ballistic charts, I can assure you that I'd not take the time to calculate the figure. Fortunately, the Oehler equipment calculates the SD for each string with the push of the summary button. Regardless, keep in mind that the lower the SD, the better.
The Kind & Knox gelatin blocks measure 8x6x16 inches and cost nearly $30 each. The gelatin can't predict what a bullet will do when it strikes metal or glass in a street fight, or bone in a hunting animal, but it shows ideally how much the bullet will expand and how much weight it will retain.
I'll also list the average instrumental energy for a round, and if measuring velocity at the target, I'll also give the energy at the target. Energy is a function of bullet weight and velocity and is expressed by velocity squared times bullet weight (in grains), divided by 218,225, divided by two. Square the velocity, multiply it by the bullet weight, and divide by 436,450. Once in a while I like to give the power factor for a round, and this is a function of bullet weight times velocity, divided by 1,000.
Hunting rounds also get a number called the Taylor Index. Concocted by famed African hunter John Taylor, this index refers to the way a cartridge performs in the field on big game. This number is calculated by multiplying the bullet's caliber by its weight times the velocity and dividing the resulting number by 7,000 (grains to the pound, in case you're wondering). Taylor had things about right, and I often find that his formula explains well why the .454 Casull seems to be more lethal in the field than are many high-powered rifles (at handgun ranges, of course).
The way a bullet flies through the air is great stuff, to be sure. It's always nice to see a round go exactly where it's directed, and it takes a good bullet to do that. Seldom, however, is that the only measure of bullet performance, unless I'm only shooting at paper. If I'm paper punching, I concern myself with accuracy, ability to function and cost. An accurate, cheap load that works gets the nod--if there's such a thing. In this line of work, I tend to think that the true measure of a round consists of a mix that includes accuracy, velocity and the power that results from it plus adequate penetration.
I strongly feel that the mix should also include the way a bullet performs in a medium that approximates what you'd encounter on the street as well as in the field. I got started handgun hunting quite some time ago. Early in my hunting career the .30 and .357 Herrett were accounting for some spectacular results in the field. Had sheer energy and potential accuracy been the only important factor, they were winners, regardless of the chosen bullet. Once in a while, however, things didn't work out quite so well when bullets that were designed to work well at 1,000 fps or less were hitting game at very nearly twice that velocity in some cases. I once saw a whitetail buck take a hit from a friend's .357 Herrett at less than 50 yards and go down like a stone, only to jump up and escape onto ground we couldn't hunt because the bullet detonated on the shoulder and didn't make the boiler room! The wound was massive and shallow.
Predicting how a bullet will perform in the real world is risky business. There are too many variables that enter the equation. Throw in heavy clothing, metal or glass on the street, or a twig in the field and all bets are off. Still, it is possible to compare loads by blasting away at carefully prepared ballistic gelatin blocks at a known and uniform range. If nothing else, I'll end up with a relative measure of past bullet performance in ballistic gelatin.
I rely on 8x6x16-inch blocks of gelatin prepared from Kind & Knox powder. Ideally, the mixture is 10 percent gelatin, 90 percent water that is blasted when it is exactly 39.2 degrees F (4 Centigrade). I've found that if I need a slightly higher percentage of gelatin, I carefully measure nine liters of my hottest (140 degrees F) tap water into a clean, white plastic bucket and slowly add 1,100 grams of gelatin powder to the water while using a plastic paint mixer on a power 3/8-inch drill to mix things. My wife, Cindy, slowly adds the powder while I run the mixer.
The author's calibration procedure calls for a .177 BB to penetrate gelatin when launched at 590 fps. Johnston uses a Daisy Model 880 that shoots within 7 fps consistently on the third shot.
I pour the hot, liquid mixture into custom aluminum molds made from .032-inch sheet aluminum. The clean molds are coated with a thin coating of PAM non-stick oil before they are filled with the mixture. When the mold has been filled, it is set aside where it cannot be contaminated for 4-5 hours and cooled to room temperature. This allows the air that was captured in the mixing to escape. It's impossible to mix things thoroughly without introducing air bubbles, but it helps to add a couple of drops of concentrated cinnamon oil to the hot water before mixing.
Then the molds are transferred to the refrigerator in the farm shop. (Ballistic gelatin spoils rapidly at room temperature.) I have yet to see an affordable refrigerator that will hold exactly 39.2 degrees, nor have I found a way to hold a block uniformly at 39.2 degrees. The acceptable measure of a properly mixed and calibrated block of gelatin calls for a .177-caliber ground steel BB to penetrate 3.34 inches (8.5 cm) when launched at 590 fps. I've found that my Daisy 880 will launch the third BB from a string at 590 fps or so within a couple fps with seven pumps and that it'll routinely get very close to the required depth when I use the 11 percent mixture. I use a dial caliper to measure the penetration of the BB and try to launch the shot "for record" within seconds of the shot from the Daisy air rifle, assuming that the Daisy hit the mark. The gelatin can't be shot until it's cooled for at least 48 hours, and I don't let it sit for longer than five days or so before it's blasted.
It would be great to be able to run the shots through twigs, wallboard, glass and metal and then into a bunch of blocks of gelatin. Unfortunately, I end up getting about 10 blocks of gelatin from $280 worth of Kind & Knox powder, and we get one shot per block unless a bullet penetrates less than 8 inches. It's not within reach of the average writer to blast merrily away at dozens of $30 targets for an article. That would be a direct line to the poor house to be sure, while the time invested would likewise be unmanageable, to boot.
Be that as it may, when the smoke has cleared, I end up knowing how this lot of ammunition performed on paper, what ballistics it generated and how it performed, relatively speaking, in ballistic gelatin. While I ca
n't accurately predict exactly how it may work in a specific situation, I do my level best to come up with a yardstick that you can take to the bank.
I hope you've enjoyed learning how I do our thing out here in the boondocks and that the next time you see my work here, you'll have some measure of the effort that goes into each piece. I still encounter locals who don't think a writer really works. Granted, it's fun, but it's work just the same.